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4 - 3 Internet Services

Various services are provided over the Internet. The typical Internet services are as described below.

(1) Electronic mail (e-mail)

An electronic mail is an electronic letter that is sent to a specific partner. In addition to text (i.e., characters) data, image data and voice data can also be sent by using a protocol called MIME (Multipurpose Internet Mail Extensions). MIME uses an encoding method called BASE64 in which data is separated into six-bit portions and converted to 64 types of alphanumeric characters. An electronic mail that makes use of the Internet is also called an “e-mail.” When an e-mail is sent, the e-mail is first sent to the mail server to which the mail client of the transmitting side is connected. The mail server determines the destination mail server from the mail address and selects the most appropriate path. (In certain cases, the e-mail may pass through another mail server before the e-mail reaches the mail server of the other party.) When the e-mail reaches the mail server of the other party, the mail client of the receiving side can acquire the e-mail from the mail server at any desired timing. (Since the e-mail is sent through the store-and-forward switching method, the sending order and the receiving order may differ when there is a transmission delay.) The e-mail transfer protocol SMTP (Simple Mail Transfer Protocol) is used to transfer an e-mail to the mail server, or to transfer an e-mail between mail servers. On the other hand, in order to acquire an e-mail from the mail server, mail reception protocols such as POP3 (Post Office Protocol version 3), which acquires the entire information within the mail server in a batch, or IMAP4 (Internet Message Access Protocol version 4), which enables the selection of the e-mail by initially acquiring only the header information, are used. IMAP4 is effective in e-mail usage in a mobile environment. · 3 questions

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After an e-mail is sent, the e-mail may be returned along with a message “Returned e-mail: … unknown”. In such a case, if “Host” is unknown, check the portion after the @ mark of the mail address because the “mail server of the destination is not found,” and if “User” is unknown, check the portion before the @ mark of the e-mail address because the “the user of the destination is not found.”

(2) Web / WWW (World Wide Web)

Web/WWW is a mechanism (i.e., service) that enables the acquisition of information (e.g., web pages) that is registered on the web server by an individual or company through a simple operation. In most cases, the web pages that are published on the web server are created by a markup language, such as HTML and XML. A web page is hypertext (or hypermedia) in which reference information (hyperlink) that can be accessed on the related web page is embedded. Therefore, in order to acquire a web page from the web server, the web client uses HTTP (HyperText Transfer Protocol). The acquired web page can be browsed through with the help of software called a browser that exists in the web client. Generally, a URL (Uniform Resource Locator) is used to reference a web page. The URL is configured by the “Protocol name://Host name + domain name/File name” (e.g., “http://www.its.co.jp/map.html”). The URL shows the location of the files that are accumulated by the web server on the Internet, and is used through conversion to the IP address.

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(3) Search engine

A search engine is a program that performs the search process on a search site that is a collection of web pages for searching information. The information search service that is provided by the information search site is included as one of the web services. The mechanism of information search that is provided by the search engine is broadly classified into three types.

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(4) File transfer service

The file transfer service is a service that downloads files to the client from an FTP server, and uploads files to the FTP server from the client. The URL is used for specifying the address in the file transfer service as well. However, since FTP (File Transfer Protocol) is used as a protocol, the portion indicating the protocol name of the URL becomes “ftp”.

(5) Other services

The following services (or utilization methods) that make use of Internet technologies are also available.